Field of the Invention
The present invention relates to a piezoelectric thin film element, an inkjet recording head having the piezoelectric thin film element, and an inkjet image-forming apparatus having the inkjet recording head.
Description of the Related Art
An image recorder or an image-forming apparatus such as a printer, facsimile machine, or copier includes an inkjet recorder provided with an inkjet recording head as a liquid discharge head (liquid drop head). Two types of inkjet recording heads (ink drop head) are in practical use, for example, an inkjet recording head using a piezoelectric actuator of a longitudinal vibration mode which extends and contracts in the axis direction of a piezoelectric film (hereinafter referred to as a piezoelectric body or piezoelectric thin film), and an inkjet recording head using a piezoelectric actuator of a flexural vibration mode.
The inkjet recording head using the piezoelectric actuator of the longitudinal vibration mode includes a piezoelectric element (piezoelectric thin film element) in which an electromechanical conversion element is laminated on a vibration plate. The electromechanical conversion element includes a piezoelectric film (piezoelectric body) of a longitudinal vibration mode which extends and contracts (expansion and contraction) in the axis direction (longitudinal direction), and an electrode which sandwiches the piezoelectric film (piezoelectric body) from top and bottom. The inkjet recording head is configured to discharge ink in a pressurizing chamber from a nozzle as drops by pressuring the ink in the pressurizing chamber with the expansion and contraction of the piezoelectric film.
In the inkjet recording film using the actuator of flexural vibration mode, it is known that an even piezoelectric film is formed as a piezoelectric material layer over the entire surface of the vibration plate with a film-forming technique, and the piezoelectric material layer is separated into a shape corresponding to a pressure generation chamber with a lithography method. The piezoelectric element is individually formed in each pressure generation room.
For example, Patent Document 1 (JP 2004-186646A) discloses that a piezoelectric film having good crystal orientation can be obtained by forming a piezoelectric film on a noble metal electrode containing titanium deposited in a strip shape on the surface.
For example, Patent Document 2 (JP 2004-262253A) discloses that a piezoelectric film having good crystal orientation can be formed by using an MgO substrate as a substrate.
For example, Patent Document 3 (JP 2003-218325A) discloses a method of manufacturing a ferroelectric film as a piezoelectric film by forming an amorphous ferroelectric film, and then crystallizing the film with a rapid heating method.
For example, Patent Document 4 (JP 2007-258389A) discloses a film-forming process using a perovskite composite oxide (may contain unavoidable impurities) having any one of a tetragonal crystal system, orthorhombic crystal system, and a rhombohedral crystal system. The perovskite composite oxide is preferentially oriented to any one of (100), (001), and (111) planes, and forms a piezoelectric film in which the orientation degree is 95% or more.
Many of the piezoelectric films in Patent Documents 1-4 are made of a PZT (lead zirconate titanium) film on platinum. Moreover, a metal material such as Pt, Ir, Ru, Ti, Ta, Rh, or Pd is used as an electrode material. In general, platinum is frequently used because it has a strong self orientation owing to a face-cemented cubic lattice (FCC) structure of a close-packing structure, strongly orients to (111) even if it forms a film on an amorphous substrate such as SiO2 which is a material of a vibration plate, and the piezoelectric film formed thereon has a good orientation.
However, the piezoelectric film in Patent Documents 1-4 has a problem in that column crystals grow due to strong orientation, and for example, Pb easily diffuses in a base electrode along a grain boundary.
Moreover, there is a possibility that oxygen loss in the piezoelectric thin film increases over time in the operation of the piezoelectric thin film (piezoelectric film, piezoelectric body). A conductive oxide electrode is used as a supplemental source of the oxygen loss component.
In order to form a piezoelectric thin film, a conductive oxygen electrode formed just below the piezoelectric thin film is important. Ruthenium acid strontium SrRuO3 (hereinafter, referred to as SRO) includes a perovskite crystal structure which is the same as that of PZT. Therefore, it has a good bonding performance in an interface, easily achieves epitaxial growth of PZT, and has a good property as a diffusion barrier layer of Pb (refer to, for example, Patent Document 5 (JP 3249496B), Patent Document 6 (JP 3472087B), and Patent Document 7 (JP H11-195768A)).
A crystal substrate can be used as a member for obtaining a piezoelectric body having good crystallizability as described in Patent Document 2. However, an application range is generally increased when a base film having good crystallizability is used. For this reason, it is advantageous to select a film having good crystallizability as a base layer of a piezoelectric film. The crystallizability is evaluated with X-ray diffraction measurement. The index includes uniform crystal surfaces or large crystal. A large particle diameter is preferable as an external appearance.
On the other hand, Patent Document 8 (JP 2007-35883A) proposes a precise film having a relatively small crystal particle diameter. In this case, it is necessary to maintain a flat surface of a film, so that lead can be prevented from diffusing to the lower layer side from the piezoelectric film.
Moreover, a piezoelectric body having (111) orientation and (100) orientation is used as a piezoelectric body having good crystallization in which crystal axes are aligned. There is a technique in which a layer is added to the piezoelectric body side of the lower electrode, and the main orientation is controlled, in order to provide uniform orientation. As an example of the orientation control layer, a layer using a metal layer or an oxidative metal layer is used (for example, refer to Patent Document 9 (JP 2001-88294A) and Patent Document 10 (JP 2006-245141A)). There is also a technique which forms piezoelectric films having a different composition till the orientation control layer, and then forms a main piezoelectric body along the orientation.
The crystal orientation of the piezoelectric film can be controlled when deterioration in the piezoelectric body due to Pb diffusion is prevented. The piezoelectric film on an SRO oxide electrode formed on a Pt electrode or an Ir electrode has (111) main orientation.
Patent Document 8 discloses an actuator made of a titanium oxidized film crystallized on the oxidized film on the vibration plate. The oxidized film has a thickness of 10 nm or more and 40 nm or less, and a particle diameter of 10 nm or more and 20 nm or less. However, in order to improve a property of a piezoelectric film, it is considered to increase the crystallizability (in particular, the particle diameter) of the base film.
To that end, it is necessary to change a process, for example, increasing a forming temperature. The crystal particle diameter can be increased if the temperature is increased. However, an adverse effect such as a hole in a crystal grain boundary occurs. Therefore, it has been investigated as to whether or not a large crystal particle can be obtained without the occurrence of such an adverse effect.